This invention relates to a starch-based, alkaline corrugating adhesive composition which contains a selected crosslinking additive that has exceptional low levels of residual formaldehyde content and helps impart good water-resistance and viscosity stability to the adhesive.
The procedures employed in the production of corrugated paperboard usually involve a continuous process whereby a strip of paperboard is first corrugated by means of heated, fluted rolls. The protruding tips on one side of this fluted paperboard strip are then coated with an adhesive, and a flat sheet of paperboard, commonly known in the trade as a facing, is thereafter applied to these tips. By applying heat and pressure to the two paperboard strips thus brought together, an adhesive bond is formed therebetween. The above-described procedure produces what is known as a single-faced board in that the facing is applied to only one surface thereof. If a double-faced paperboard in which an inner fluted layer is sandwiched between two facings is desired, a second operation is performed wherein the adhesive is applied to the exposed tips of a single-faced board and the adhesive-coated tips are then pressed against a second facing in the combining section of the corrugator under the influence of pressure and heat. The typical corrugating process and the use and operation of corrugators in general are described in U.S. Pat. Nos. 2,051,025 and 2,102,937 to Bauer.
Starch-based adhesives are most commonly used in the corrugating process due to their desirable adhesive properties, low cost and ease of preparation.
The most fundamental of starch corrugating adhesives is an alkaline adhesive which is comprised of raw, ungelatinized starch suspended in an aqueous dispersion of cooked starch. The adhesive is produced by gelatinizing starch in water with sodium hydroxide (caustic soda) to yield a primary mix of gelatinized or cooked carrier, which is then slowly added to a secondary mix of raw (ungelatinized) starch, borax and water to produce the full-formulation adhesive. In the corrugating process, the adhesive is applied (usually at between 25.degree. and 55.degree. C.) to the tips of the fluted paper medium or single-faced board, whereupon the application of heat causes the raw starch to gelatinize, resulting in an instantaneous increase in viscosity and formation of the adhesive bond. Such adhesives are described in the above-noted patents to Bauer. Typical "no carrier" starch adhesives are described in U.S. Pat. No. 3,487,033 to McElmury et al., and U.S. Pat. No. 3,355,307 to Schoenberger et al.
It is often desired or necessary in the manufacture of corrugated paperboard that the adhesive yield water-resistant bonds which can withstand extended exposure to high humidity, liquid water, melting ice and the like. A number of approaches have been devised to produce water-resistant corrugating adhesives. One method involves the preparation of an acidic, starch-based adhesive wherein urea-formaldehyde resin is added to the composition, together with an acidic catalyst such as aluminum sulfate, to produce water-resistant bonds in the corrugated board manufactured therewith. The adhesive composition itself, however, is deficient in other important properties such as corrugator bonding speeds, viscosity stability and pot life, and exhibits excessive formaldehyde odor. In addition, acidic corrugating adhesives tend to be corrosive.
The many disadvantages associated with the acidic corrugating adhesives led to the development of water-resistant alkaline curing starch-based adhesives for use in the corrugating industry. In the preparation thereof, a thermosetting resin, such as, e.g., urea-formaldehyde, resorcinol-formaldehyde, melamine-formaldehyde, phenol-formaldehyde, diacetone acrylamide-formaldehyde, ketone-aldehyde and urea-acetone-formaldehyde condensate, is added to the adhesive as a crosslinking additive for the amylaceous components to produce water-resistant bonds. Preferred among these resins for superior water-resistant properties are ketone-formaldehyde condensates as disclosed in U.S. Pat. No. 2,529,851, and particularly acetone-formaldehyde resins. Some adhesives made from such resins, however, suffer from poor pot life and viscosity instability, as well as considerable formaldehyde odor.
In recent years, due to the toxicity of and increasing governmental regulations concerning formaldehyde, serious efforts have been made to reduce the levels of exposure to formaldehyde in the industrial workplace. Acetone-formaldehyde resins such as are employed as crosslinking additives in corrugating adhesives contain about 1.0 to 4.0% free (unreacted) formaldehyde by weight of condensate. Prior attempts to reduce formaldehyde levels in crosslinking additives as taught in U.S. Pat. Nos. 3,019,120 and 3,294,716 have not reduced free-formaldehyde amounts to a significant extent and/or have resulted in diminution of the degree of water-resistance achieved in the bonds formed.
In U.S. Pat. No. 4,366,275 to Silano et al., the crosslinking additive used with the starch-based alkaline corrugating composition comprises a mixture of acetone-formaldehyde condensate and dimethylol dihydroxy ethylene urea (DMDHEU) wherein at least a portion of the DMDHEU present is produced "in situ" by reaction of the free-formaldehyde contained in the acetone-formaldehyde condensate with dihydroxy ethylene urea. The patent discloses that the unreacted formaldehyde in the acetone-formaldehyde resin condensate is reduced to about 0.1 to 2% by weight. Experience has shown, however, that in most instances the free-formaldehyde is reduced only to a level of about 0.5 to 0.9% by weight of the condensate. Current industry requirements call for still lower levels of unreacted formaldehyde.
A recent patent, U.S. 5,079,067 to Willging, discloses the reduction of free-formaldehyde in formaldehyde containing resins to a level of less than 0.3%, by weight (of aqueous resin composition), by reacting the free-formaldehyde with a nitrogen base and urea in the presence of an acid catalyst.
Another recent patent, U.S. 5,247,066 to J. Schoenberg et al., discloses another method for reducing levels of free-formaldehyde in ketone-formaldehyde crosslinking additives by treating the unreacted formaldehyde with hydrogen peroxide. This method has resulted in significant reduction in free-formaldehyde content to levels of less than about 0.4% by weight of condensate (i.e., aqueous condensate or solution).
While these methods generally provide lower free-formaldehyde levels than previously attained, they do not always provide the water-resistance, viscosity characteristics and ease of process conditions that are desired. Furthermore, there is a need and desire to develop corrugating adhesive compositions which while providing suitable water-resistance, also have reduced levels of residual formaldehyde and particularly are viscosity stable.
It has long been known to use sulfite salts to reduce formaldehyde content in different non-related technical applications such as textiles, plywood/particle board manufacture, cosmetics and paper production. However, it has heretofore not been known to use ketone-formaldehyde additives treated with selected sulfite salts in corrugating adhesives to provide compositions with good water-resistance and particularly improved viscosity stability while also providing low levels of free formaldehyde content in the additive.